Metal parts of various industrial machines or general-purpose machines are required to have various properties such as impact resistance, corrosion resistance and wear resistance depending upon their respective purposes. However, in many cases, the metal (or substrate) constituting such metal parts cannot adequately satisfy the required properties by itself, and it is therefore often subjected to surface modification, particularly by forming a coating or deposition on the substrate surface.
Varieties of powder compositions for different deposition processes are known and described in publications. Mainly, the powders comprise particles of a nonmetallic/ceramic compound like tungsten carbide, chromium combined with a metal such as Ni, Cr, Co or an alloy containing such a metal as a binder, to form a ceramic/metal composite material. Conventional composite materials based on metallic and nonmetallic compounds are presently made by different manufacturers like Praxair, Sulzer-Metco, etc. A very brief list of some of these compounds is the following: WC-12Co; WC-17Co; WC-10Co-4Cr; Cr3C2-25NiCr; WC-10Ni. The metallic component of a composite metal-ceramic (cermet) is in a thermodynamically stable state and represented by a metal or an alloy having a crystalline structure.
More complex compositions of cermet powders for surface modification are also well known. Such compositions, comprising nitrides, carbonitrides, borides (for example, titanium carbonitride) and multicomponent metallic alloys (for example, CoCrAIY, FeCrAIY, and so on) have not yet found wide application.
Thermal spray coating typically follows one of several general schemes disclosed in literature and patents. In a first method, particles used to coat a substrate may be heated so that their temperature when they contact the substrate is greater than their melting temperature. This case is generally typical for conventional flame spraying, atmospheric plasma spraying (APS) and low pressure plasma spraying (LPPS). Because the particle is in a melted or fused state and traveling at a relatively high velocity when it contacts the substrate, splashing of the melted or fused particles often occurs during the collision and interaction with the substrate. Melting of the nonmetallic component of a composite powder may cause its undesirable decomposition and phase transformation as well.
As shown in FIG. 12, a normal component of pressure may exist only on a surface area underneath the particle having a diameter Dx, which is equal or smaller than diameter D of the sprayed particle. In this case, good bonding may only develop in the area with diameter Dx underneath the particle. The particle will splash, and the portion outside diameter Dx, may not make proper contact with the substrate to enable good bonding to it. Splashing often results in voids and excessive surface area of the splashed particles. These characteristics may, consequently, result in excessive oxidation, low wear, corrosion and erosion resistance of the coated object. Higher impact velocities may result in the higher intensity of the impact, splashing, and a greater area of the particle extending outside of diameter Dx.
According to a second method, thermal spraying may take place under conditions such that ETp+Vp2/2>Em, where ETp+Vp2/2 is the total energy of the particle upon collision with the substrate; it is the thermal energy of the particle upon contact with the substrate. Em is the energy needed to heat and melt all the components of the particle (the latent heat of melting is included in Em). Tp is the temperature of the particle upon contact with the substrate. Vp is the velocity of the particle upon contact with the substrate, and Tm is the melting temperature of the particle's nonmetallic component. Such a scheme is often termed “impact fusion” and was disclosed in U.S. Pat. No. 5,271,965, which is incorporated by reference. While the intensity of splashing may be lower than that experienced in the first spraying scheme, splashing of the coating particles is generally experienced during impact fusion. Splashing that occurs during impact fusion results in all of the same consequences discussed above.
According to a third coating method, particles may be heated to a temperature sufficiently low to prevent thermal softening of sprayed particles. The particles heated in this manner may be applied to a substrate at high velocities. This coating scheme may generally only be applicable for use with coating materials having very low yield stress, for example, in a general range of about 200 MPa or less. Amorphous and nanocrystalline alloys considered in the disclosure have significantly higher yield strength which is in the range of about 500-1200 MPa at room temperature. However, this third coating scheme may have a very low efficiency when spraying the metal-ceramic composite powders.
Disadvantages of prior art composite materials are partially related to the metallic compounds or binders, which are based on metals like Ni and Co, and the conventional crystalline alloys with other metals like Cr, etc. In some cases these types of binders do not provide desirable corrosion resistance, bonding with nonmetallic components of a composite material, wear and erosion resistance, toughness and some other properties determining performance of the composite material. Notably, Co and Ni based alloys are expensive. The current deposition processes allow one to deposit the conventional metal-ceramic composites listed above, as well as amorphous and nano- or microcrystalline metallic alloys, more or less successfully.
However, there is still a need for novel metal-ceramic composite coatings and depositions containing amorphous and nanocrystalline metallic components.